Some fixed IPs have no Internet Connection

I got some weird issue.
Some days ago my old router died. The IP Adress of my old Router was 192.168.15.1.
And since my old router had the same IP i didn't expect to have issues.
However i don't have internet access on my Fathers PC anymore.

However i only seem to get internet on his machine when i change his IP to 192.168.15.6
If i change it to 192.168.15.2-192.168.15.5 no internet again. I wonder how that is even possible since we only have 2 PCs connected to the router?